Homing pigeons return to their lofts from unfamiliar release sites. We are investigating the effect of altering the magnetic field to which the bird is exposed. The field is altered both during the trip to the release site and also during the homeward trip. In each case the bird is tracked by radio telemetry and its homeward journey compared with that of control birds who experienced the normal earth's magnetic field. It now appears that under overcast conditions when the sun is not visible to the pigeon that a field of 0.6 Gauss will upset the pigeon's orientation. If the polarity of the applied field is such that a compass needle inside the coil surrounding the pigeon's head has its North seeking needle pointing up (NUP), the birds often fly 180 degrees away from the correct direction to home. A similar coil of the opposite polarity (SUP) has no effect. Transporting the birds to the release point inside a fluctuating magnetic field impairs their orientation. We are now beginning to explore the mechanisms responsible for this effect. BIBLIOGRAPHIC REFERENCES: Walcott, C., 1974 The Homing of Pigeons, American Scientist 62: 543-552. Walcott et al., 1975 Reactions of Migrating Birds to Lights and Aircraft, Proc. Nat'l Acad. Sci. 72: 1994-1996.